Back Analysis and Closure Considerations for the North-East Pit Wall at the Globe Progress Gold Mine, Reefton, New Zealand

In April 2015 a kinematic slope failure occurred on the north-east (NE) pit wall at the Globe Progress gold mine in Reefton, New Zealand. The failure involved planar sliding, toppling and translational slumping through the entire height (220 m) of the wall. A probabilistic back analysis using Rocscience’s 2D limit equilibrium software, Slide, was undertaken to determine rock mass strength parameters that led to failure. Back analysis results were then carried into a post closure model, also using Slide. The post closure model examines how the stability of the NE pit wall will respond to increasing pit water levels as the mine is transitioned into a pit lake. The back analysis found that to replicate the April failure, the greywacke – argillite bedding planes required an apparent cohesion of 32 kPa and a friction angle of 28°. It is interpreted that these strength parameters are a result of a period of heavy rainfall prior to failure. Post closure modelling concluded an increase in pit water level is not expected to induce instability of the NE wall, given no other external conditions occur in conjunction with pit water rise.